Transfer matrix method — resonant tunneling when quasi-bound state aligns with incident energy
Two rectangular barriers create a quasi-bound well state. When the incident energy matches a resonance energy, transmission T → 1 even though E < V₀.
This is the operating principle of resonant tunneling diodes (RTDs). The transmission peaks are Breit-Wigner resonances with width Γ ∝ e^{-2κd}.
Above-barrier transmission also shows oscillatory structure from interference.